1. Field of the Invention
The present invention relates generally to methods and devices of flattening and broadening the pass band spectra of optical elements and devices that require flat and wide wavelength pass band, and, in particular, to methods and devices of flattening and broadening the pass band spectrum for grating-based optical components used for transmitting and receiving laser light through a single-mode optical fibers of multi-channel optical communications networks.
2. Status of the Prior Art
Fiber optic networks are becoming increasingly popular and important for high-speed and large capacity data transmission. The networks are continuously growing due to the explosive expansion of telecommunications and computer communications, especially in the area of the Internet. This has created a dramatic increase in the volume of worldwide data traffic and has placed an increased demand for communication networks to provide increased bandwidth. To meet this demand, fiber-optic (light wave) communication systems have been developed to harness the enormous usable bandwidth (tens of tera-Hertz) of a single optical fiber transmission link. Because it is impossible to exploit all of the bandwidth of an optical fiber using a single high-capacity channel, wavelength division-multiplexing (WDM) fiber-optic systems have been developed to provide high-capacity transmission of multi-carrier signals over a single optical fiber thereby channelizing the bandwidth of the fiber.
In accordance with WDM technology, a plurality of superimposed concurrent signals are transmitted on a single fiber whereby each signal has a different wavelength. WDM technology takes advantage of the relative ease of signal manipulation in the wavelength, or optical frequency domain, as opposed to the time domain. In WDM networks, optical transmitters and receivers are tuned to transmit and receive on a specific wavelength such that many signals operating at distinct wavelengths share the single fiber.
Wavelength multiplexing devices are commonly used in fiber-optic communications system to generate a single multi-carrier communication signal stream in response to a plurality of concurrent signals each having different wavelengths and received from associated sources or channels for transmission on the single fiber. At the receiving end, wavelength division demultiplexing devices are commonly used to separate the composite wavelength signal into the several original signals each having a different wavelength.
Some of the most important components in the wavelength division system are demultiplexers, multiplexers, optical/add/drop multiplexers, and wavelength selective switches. It is advantageous to have wide wavelength pass bands for these components without degrading the signal performance and increasing the insertion loss. Although the operating wavelength for each of the transmitter lasers is tuned to the ITU grid wavelengths as close as possible when it was manufactured, there is always some offset to the ITU wavelength grid. Accordingly, the wider the pass window (i.e., pass band), the more tolerant is the laser offset specification such that it is easier to adjust the system. Also, there is always some drift, both in terms of the laser center wavelengths and the center wavelength of the pass band itself, such that the wider pass band allows the system to be more tolerant so that the center wavelength can ‘walk out’ the ‘passing window’ of the demultiplexer. Furthermore, the wider the pass band, the flatter the pass window will be. Therefore when many components are cascaded in series, the total pass band shape will not deteriorate quickly and the signal can travel farther without re-conditioning.
In free-space grating-based devices, such as multiplexers, demultiplexers, optical add/drop multiplexers, wavelength-selective switches, etc. . . . , either transmissive or reflective diffraction gratings are employed as spectral dispersion elements. The position of a given spectral component is a function of the diffraction angle. For example, in a narrow spectral range (C-band, L-band, or C+L-band), the geometrical separation between two neighboring ITU channels is approximately equal. Single-mode fiber arrays are used to couple the diffracted light field and transmit individual channel signals. The spectral response near the pass band portion outputted from a given single-mode fiber is substantially Gaussian with a “narrow” bandwidth. Such a spectral shape is not desirable and it would be better to broaden and flatten the Gaussian pass band spectra.